1. Field of the Invention
This invention relates to engravers and, more particularly, to an engraving system and method having the capability of engraving a plurality of engraved areas on at least one cylinder using a plurality of engraving heads where the engraved areas define differing screens.
2. Description of the Related Art
This invention relates to engraving of cylinders commonly used in the gravure printing process and specifically to engraving apparatus and methods of the general type disclosed, for example, in U.S. Pat. No's. 4,357,633; 4,438,460; 4,450,486; 4,451,856; 4,500,929; 5,029,011; 5,293,426; 5,402,246; 5,416,597; 5,422,958; 5,424,845; 5,424,846; 5,438,422; 5,440,398; 5,454,306; 5,475,914; 5,491,559; 5,492,057; 5,493,939; 5,519,502; 5,555,473; 5,583,647; 5,617,217; 5,621,533; 5,652,659; 5,661,565; 5,663,802; 5,663,803; 5,671,064; 5,675,420.
The basic principle of electromechanical engraving of the gravure cylinder involves rotating a copper plated cylinder or cylinders while actuating an electronically driven tool or stylus which cuts or engraves vertical columns or lines into the copper surface. The engraved cylinder is normally used in a web type printing press for printing paper, plastics, metallic film material and the like.
In the field of gravure printing, the majority of printed images are screened halftone images. The screened image contains a geometric pattern of dots to represent different gradations of a continuous tone image. As with other forms of halftone gravure, the screen refers generally to the number of cells per square centimeter (i.e., Cell/cm.sup.2). When four geometric patterns or screened images, one for each processed color, are superimposed upon each other in register they create a final geometric pattern which, when printed, results in a high-quality image.
The size of any particular engraved area or cell vertically along the direction of cutting (i.e., a cell height) and the size horizontally in the row-to-row direction (i.e., a cell width) are each selected by a user in order to achieve the desired screen and screen angle. Alternatively, the user of a gravure engraver may directly specify both the screen angle and screen (and, hence, the cell parameters, such as width and height) for each color being used in order to alter the screen rotation and population or density of engraved areas or screen ruling.
By properly aligning the screens, a tight geometric configuration and improved printing is achieved. If the screened separations are not rotated at the proper screen angle, a moire pattern or plaid image effect occurs in the printed image.
Typically, in a four color printing process, a minimum of four gravure cylinders, one for each color, are required. In order to facilitate eliminating undesired moire effects, each cylinder of a multi-color printing set was engraved with either a different screen and/or different screen angle so that when the patterns overlap moire effects were reduced or eliminated altogether.
The Helio-Twin.TM. engraving system offered by Linotype-Hell of Germany, as well as the engraving system model no. B722 offered by Ohio Electronic Engravers, Inc. of Dayton, Ohio, provided engraving systems for simultaneously engraving multiple cylinders. One drawback of these systems was that the engraving heads used to engrave each cylinder were mounted on a common carriage or leadscrew which was driven such that all engraving heads moved across its associated cylinder at the same rate of speed.
Another drawback of the systems offered by Linotype-Hell and Ohio Electronic Engravers is that the cylinders were rotated at the same speed and the heads frequencies were not independently controllably. This, in turn, made it difficult to simultaneously engrave multiple patterns having different vertical screen rulings. Thus, the vertical resolution defined by the vertical screen and the horizontal resolution defined by the horizontal screen for each engraving head were not independently and selectively controllable.
One approach to overcoming the problems with multiple head engraving systems was to vary the frequency of each engraving head. Unfortunately, this only provided the ability to change the vertical resolution or vertical screen for a single head. For example, FIG. 9 illustrates two screen sets (A,B) and (C,D) which illustrate the type of screens engraved with prior art, multi-head engraving systems. In this illustration, screens A and B are engraved on a single engraver by varying the frequency of one of the heads on the engraver. Similarly, C and D are also engraved on a single engraver. Notice that the vertical heights VHA, VHB, VHC, VHD were variable. However, because the engraving heads were driven at the same feed or slew rate, the width HWA associated with screen A was the same as the width HWB associated with screen B and vice versa. Likewise, the width HWC associated with screen C was the same as the width HWD associated with the screen D.
Unfortunately, the area or density of each engraved area represented by screens (A & B) or (C & D) must be different for each screen angle. The engraving heads were not independently controllable on multi-head engravers such that the screen could be kept constant, while the screen angle was varied or vice versa. In addition, the multiple heads were not independently controllable so that the areas or densities engraved by the heads were constant while a screen angle defined by areas engraved by the first head were different from a screen angle defined by areas engraved by the second head.
What is needed, therefore, is a system and method which overcomes the problems of the prior art and which gives the user the ability to selectively and independently engrave a plurality of engraved areas defining different screen rulings and/or wherein the vertical resolution or vertical screen and horizontal resolution or horizontal screen vary between the engraved patterns, despite the fact that the engraving heads are engraving one or more surfaces while the surfaces are rotating at the same speed.